The present invention relates to a vehicle steering apparatus and a control method for the apparatus.
In recent years, steering control systems have been proposed which control the steering angle of steered wheels for controlling the yaw moment of a vehicle based on a vehicle model (vehicle motion model) obtained by modeling the relationship between the motion state of the vehicle and the vehicle state quantities such as the vehicle speed and the yaw rate (for example, Japanese Laid-Open Patent Publication No. 2002-254964). One of such steering control systems is the intelligent front steering (IFS) system.
The IFS system executes an integrated electronic control related to steered wheels, such as variable gear ratio control, in which the transmission ratio (gear ratio), which is the ratio of the steering angle of the steered wheels to the turning angle of the steering wheel (steering wheel turning angle), is varied based on the vehicle state quantities. The IFS system determines the steering characteristic of the vehicle based on vehicle model calculation, and controls the steering angle of the steered wheels to stabilize the attitude of the vehicle according to the steering characteristic.
For example, when the vehicle is in understeer, the steered wheels are controlled such that the actual steering angle by which the steered wheels are steered is reduced (understeer control). Thus, the vehicle attitude is stabilized on a low μ road, on which the vehicle attitude is likely to be unstable (for example, refer to Japanese Laid-Open Patent Publication No. 2004-1590).
The vehicle steering apparatus disclosed in Japanese Laid-Open Patent Publication No. 2004-1590 determines whether the understeer control should be executed based on the degree of grip of the steered wheels. However, when estimating the degree of grip of the steered wheels, the steering torque must be detected. Thus, hydraulic power steering apparatuses, which are widely used, need to be equipped with a torque sensor that electrically detects the steering torque. This complicates the configuration and increases the manufacturing costs.